1. Field of the Invention
The present invention relates to a control method of embedding an electronic watermark in image data of multi-valued gradation.
2. Description of the Related Art
In recent years, the prevalence of digital data has increased with the global spread of networks. In such a situation, electronic watermarks are watched from the point of view of copyright protection. The “electronic watermark” is a technique of embedding information in digital data so that a human being cannot perceive the information. By using the technique, copyright management of content data and the prevention of unjustified use thereof are performed.
By using the electronic watermark, the following information can be embedded in image data:
1. copyright information,
2. use regulation information (prohibition of copying, etc.),
3. owner indicating information,
4. historical management information of an inter protocol (IP) address,
5. data information (date and time, serial number, etc.), and
6. user designation information (user input information).
Various electronic watermark techniques have been proposed lest a human being should be able to perceive the electronic watermarks. A method of embedding an electronic watermark in low-order bits in a space area in which image data is configured is called a “space area type”. A method of embedding an electronic watermark in the high frequency side of transformed data obtained by frequency transformation of image data is called a “transformation area type”. These methods utilize the property of a human being such that the perception of the human being is sensitive in a low frequency band and is insensitive in a high frequency band.
However, the electronic watermark embedded in a low-order bit of image data or a high frequency band thereof is easily removed by the execution of a low pass filtering process and a smoothing process, and the electronic watermark has a weak resistance property. If the electronic watermark is removed in this manner, then the deterrent force of the unjustified use is gone in the case of using the electronic watermark for the purpose of copyright protection.
A method of embedding an electronic watermark into the frequency components important for the perception of a human being for the compensation of the weak resistance property has been proposed (see, for example, Japanese Patent Application Laid-Open No. H09-191394). Because the method embeds the electronic watermark by spectrum spreading, the watermark itself is recognized as additional random noises.
However, the method proposed in the Japanese Patent Application Laid-Open No. H09-191394 has a problem of image quality deterioration because the electronic watermark is embedded in the frequency components important for the perception of a human being.
As described above, the conventional techniques have the problems of the weak resistance property in the case of embedding the electronic watermark so as to reduce the image quality deterioration and of the increase of the image quality deterioration in the case of embedding the electronic watermark so as to strengthen the resistance property. That is, the conventional techniques have the problem that the improvement of the resistance property of the electronic watermark and the prevention of the quality deterioration are mutually exclusive.